1. Field of the Invention
The present invention relates to surface contamination detection. More particularly, the invention relates to photoemission detection of surface contamination on mirrors or masks used in EUV (extreme ultraviolet) technology.
2. Description of Related Art
U.S. Pat. No. 4,998,019 to Stokowski et al., which is incorporated by reference as if fully set forth herein, describes contaminant detection on the surface of a variety of electrically conductive materials (e.g., semiconductor surfaces, metals, or metal silicides). FIGS. 1 and 2 depict an example of a photocontamination detection scheme using UV light source (15, 35), focusing optics (17,37), and set of electrodes (41) to pick up photoemission from the test surface. Positive bias supplies (43, 50) and set of electrodes (49) provide correction for capacitive changes with gap dimension. Operational amplifiers (45, 52), differential processing (55), and a mechanical scan method (13) allow coverage of all points on the test surface.
The photocontamination detection scheme disclosed in U.S. Pat. No. 4,998,019 is intended for use at pressures up to atmospheric pressure with the electrodes running close to the test surface to minimize electron attachment and loss of sensitivity (mainly due to O2 presence). Electron attachment and the loss of sensitivity could also be minimized by introducing gases such as He, Ne, Ar, Kr, Xe, or N2 into the optional electrical shield (59). The disclosed scheme may be very sensitive with the photoelectric current being reduced by up to four to six orders of magnitude by an increase in the contamination layer thickness of 100 Å. The disclosed scheme is a stand alone test, however, that requires a dedicated UV light source and close location of the electrodes to the test surface in order to maintain sensitivity. The close location of the electrodes may interfere with the primary EUV beam path and may require the second set of electrodes to compensate for capacitance gap-related changes, which increases the design complexity of the system.
Several techniques for contamination detection using photoelectric emission detection have been subsequently developed for EUV lithography (EUVL). These techniques can be used to not only monitor contamination and radiation flux but to also maintain system optical alignment, control EUVL exposure levels, and regulate cleaning processes. Examples of these techniques may be found in U.S. Pat. Appl. Pub. Nos. 2002/0190642; 2007/0008517; and 2009/0059196; and U.S. Pat. Nos. 6,710,351; 6,545,272; 6,842,500; 7,060,993; and 7,928,412, all of which are incorporated by reference as if fully set forth herein. Some problems with these techniques include invasive features such as requiring electrically isolated and biased EUVL mirrors with surface contact or employing detectors built into the structure of ML (multilayered) mirrors. Other problems include constraints in the space envelope with arranged detectors at pre-defined angles to the target, unknown/unspecified sensitivities due to using unspecified ammeters to monitor photoelectric currents, required structure and support by using electrodes in forms of rings directly above mirrors to monitor photoelectric currents, required gas supplies and regulation by introducing gases into the EUV vacuum system to emit photoelectrons in situ along the EUV path, and/or having unspecified detectors and unidentified means of coupling radiation or photoelectrons.
Many other techniques have also been used for detecting surface contaminants including, but not limited to, other forms of photoemission spectroscopy (e.g., angle-resolved photoemission spectroscopy (ARPES)), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and Auger electron spectroscopy (AES). Such techniques are capable of detecting surface contaminants at extremely low levels. These techniques, however can be costly, have conflicting space envelope issues, have a large footprint, cause surface damage, and/or be difficult to integrate into EUV mask inspection systems. The conflicting space envelope issues arises due to many of the surface contamination detection systems not being able to easily fit into the space envelope of EUV mask inspection systems. Additionally, drive, source, control, and other ancillary equipment may require considerable additional footprint.